1. Field of the Invention
This invention relates to a non-aqueous electrolyte secondary cell and a method of manufacturing the same, the cell having a negative electrode of carbon material to which intercalation by charging and deintercalation by discharging of lithium are possible.
2. Description of the Prior Art
In recent years, electronic appliances have become more and more portable or cordless. In such circumstances, demand for the development of secondary cells which are smaller, lighter and of higher energy density by manufacturers of such appliances are increasing.
Non-aqueous secondary cells with negative electrodes of metallic lithium or lithium alloy can produce high voltages and high energy densities and, hence, research and development of these various types of battery systems have been active. However, it was found that discharging capacity decreases as charging and discharging are repeated in cells with negative electrodes made of metallic lithium which seems to be capable of producing the highest voltages and highest energy densities.
The decreasing discharging capacity is likely due to internal short circuiting of the cell by dendritic metallic lithium deposited on the negative electrode or side reactions of the decomposition of the organic solvent in the non-aqueous electrolyte. Also, such cells were insufficient in high-rate charging and discharging characteristics and in over-discharging durability, and the application in other fields was considered difficult. Further, suspicions of poor safety of cells having negative electrodes produced of metallic lithium or lithium alloy were big obstacles for putting them to practical use.
Thus, instead of negative electrodes of metallic lithium or lithium alloy, a new type of negative electrode material has been discovered which utilizes intercalation and deintercalation reaction of lithium by charging and discharging.
Among them, using carbon material as the negative electrode has been produced. Carbon material has nearly the same characteristic as metallic-lithium used in negative-electrode. For example, see U.S. Pat. No. 4,423,125. And, it has been reported in research that carbon materials are capable of intercalating or deintercalating more lithium by charging and discharging thereby obtaining negative electrodes of higher capacity.
Of the reports, two main themes are seen: one argues that highly graphitized carbon materials, such as natural graphite or artificial graphite are suitable; the other reports pseudographite materials such as those obtained by carbonization of hydrocarbons or polymer materials at temperatures as low as 1000.degree. to 1500.degree. C. are suitable.
In both cases, various points have been discussed, e.g., the effects of using different kinds of precursor carbon, the methods and condition of carbonization and graphitization. Interplanar spacing values of (002) plane or crystal thickness along the c axis or a axis obtained by X-ray powder diffraction for carbon materials have been reported.
It has been known that the quantity of lithium in the intercalation compound which has been formed by intercalation between layers of graphite is at a maximum when it has the form of C.sub.6 Li. The specific capacity of graphite at this case being 372 mAh/g.
However, a pseudographite material with low graphitization has a low quantity of intercalated lithium, so that the specific capacity is as low as 200 mAh/g, limiting the capacity of the cell.
In order to solve the above problems and to obtain high capacity, a method has been proposed and described in U.S. Pat. No. 5,344,724 by some of the inventors of the present invention. According to that method, pitch is melted to produce mesophase carbon micro beads, which are the intermediate products between liquid phase and solid phase, the beads are then carbonized and then graphitized to become mesophase graphite and are then used as the negative electrode material. By using mesophase graphite particle for the negative electrode, the quantity of intercalated lithium is increased, resulting in the increased specific capacity of the negative electrode. However, when negative electrodes are prepared by coating core foils of copper or stainless steel (which are to become collectors) on both the surfaces with paste composed of the mesophase graphite particles and a binder; drying the electrodes; pressing the electrodes by a roller, and simultaneously fabricating cells, the cells were found to have the quantity of lithium intercalated in the mesophase graphite particles greatly reduced when they are charged at temperatures below 0.degree. C., compared with at ordinary temperatures. The lithium which was not intercalated was deposited on the negative electrode surface in a metallic dendritic form. The metallic lithium, once deposited as dendrite, is not extinguished when the charging and discharging are repeated, resulting in no contribution to the main reaction, and a decrease in the cell capacity. Such cells do not recover discharge capacity if they are then put into ordinary temperatures. Furthermore, these cells short circuit internally due to the metallic dendritic lithium and have a short charging and discharging cycle life. Such deposition of metallic lithium and decrease of cell capacity are not observed if the charge and discharge cycle are performed at ordinary temperatures.
The above described deterioration by charging and discharging at a low temperatures likely results from the basal plane of the graphite crystal becoming parallel to the core foil surface due to high orientation characteristics of the mesophase graphite. It has been found that at the surface perpendicular to the basal plane of the graphite crystal lithium is easily intercalated; whereas, in the above described negative electrode, the planes to which lithium is to be intercalated are arranged almost perpendicular to the negative electrode surface. This leads to the checking of lithium easily intercalated between the graphite layers during charging.